Sense amplifiers are commonly used in several applications, such as for reading non-volatile memory devices. A sense amplifier consists of a circuit that can measure an input signal at low level (such as a current).
A standard sense amplifier compares the input current with a reference value. For example, in a non-volatile memory device, the current provided by a selected memory cell is compared with the current provided by a reference cell. Typically, the sense amplifier includes a current mirror with unbalanced loads. A portion of the reference current provided to an input branch (generally one half of the reference current) is mirrored into an output branch connected to the memory cell. In this way, the voltage at a node of the output branch increases or decreases according to whether the current of the memory cell is lower or higher than one half of the reference current. A comparator compares this voltage with a voltage at a node of the input branch and determines the value stored in the memory cell accordingly.
However, the structure described above cannot be used in a parallel sense amplifier, wherein the input current must be compared with multiple reference values at the same time; a typical application of this sense amplifier is the reading in parallel mode of a non-volatile multilevel memory device. Indeed, in this case it is difficult to impossible to mirror the different reference currents (provided to respective input branches) into the same output branch connected to the memory cell.
A known solution is that of using a load including a diode-connected transistor in each branch of the sense amplifier (both the input branch and the output branch). In this way, each reference current is mirrored into an input terminal of a corresponding comparator, while the current of the memory cell is mirrored into the other input terminals of all the comparators. The currents being input to each comparator generate a corresponding voltage, which allow establishing whether the current of the memory cell is lower or higher than the corresponding reference current. The combination of the results of the different comparisons identifies the value stored in the memory cell.
A drawback of the structure described above is that the voltages being input to the different comparators typically have very low values (since the load transistors in the corresponding current mirrors exhibit a negligible resistance). Therefore, the sense amplifier is rather inaccurate.
Moreover, the several current mirrors consume a relatively high amount of static power. These current mirrors also introduce further inaccuracies owing to the unavoidable tolerances of the manufacturing processes.
A different sense amplifier (of the non-parallel type) is described in U.S. Pat. No. 6,128,225. In this case, the current of the memory cell is provided to the input branch of the current mirror so as to be reflected to the output branch connected to the reference cell. Nevertheless, this sense amplifier can be used only in a standard memory device. Indeed, the current mirror has unbalanced loads. Therefore, the sense amplifier is incompatible with a parallel sense amplifier (particularly, for use in a multilevel memory device).